1. Field of the Invention
This invention relates generally to a method and apparatus for forming an extrudate.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
It is known to form an extrudate using an extruder that includes a housing having a mixing chamber defined by a pair of parallel intersecting housing bores, and a pair of screw shafts supported for rotation about respective generally parallel axes and including respective screw sections positioned for co-wiping intermeshing rotation within the respective housing bores of the mixing chamber. A drive mechanism is operably attached to the screw shafts and rotates the screw shafts in the same direction and in the same sense.
For example, U.S. Pat. No. 6,042,260 issued 28 Mar. 2000 to Heidemeyer et al., discloses an extruder that includes a pair of screw shafts supported for rotation in a housing and including respective screw sections positioned for co-wiping intermeshing rotation within respective housing bores of a mixing chamber in the housing. The screw sections each have an outside diameter to inside diameter (OD-ID) ratio of “greater than or equal to” 1.55. A drive mechanism rotates screw shafts are rotated in the same direction and in the same sense at rotational speeds of “at least” 800 rpm and at a torque density of “at least” 11 Nm/cm3). To support the screw shaft for rotation, the screw sections of the screw shafts and the housing bores are sized to leave only a small screw-to-chamber wall clearance, i.e., a small gap between the screw sections of the screw shafts and the respective housing bores of the mixing chamber. However, such a small tolerance can result in significant shear heating in the vicinity of the screw sections—especially at shaft rotation speeds in excess of 800 rpm.
What would be desirable would be an extruder designed to reduce shear heating in the vicinity of the screw sections to acceptable levels, even at shaft rotation speeds in excess of 800 rpm.